This invention relates to biological water and wastewater treatment systems, and in particular, to a method for reducing the amount of nitrous oxide released to the atmosphere by such systems.
Nitrogen compounds promote unwanted growth of algae and other aquatic plants in lakes, rivers and other water resources. Consequently biological nitrogen removal (BNR) processes have become more prevalent in the treatment of wastewater. Nitrogen is removed biologically by nitrification and denitrification. Nitrification is an autotrophic aerobic process to convert ammonium into nitrate, through two successive reactions: NH4+→NO2−→NO3−. Denitrification is a heterotrophic anoxic process whereby nitrate (NO3−) is converted into gaseous nitrogen (N2). In both processes nitrous oxide (N2O) is an intermediate that is released into the atmosphere.
Nitrous oxide is a gas which is very harmful to the environment because of its warming effect on the Earth's atmosphere. Currently there is a concern for chemical compounds in the Earth's atmosphere that act as “greenhouse gases.” These gases allow sunlight to enter the atmosphere. However, when the sunlight is radiated back towards space as infrared radiation, the gases absorb the energy, thereby trapping heat within the Earth's atmosphere. According to the United States Energy Information Administration, nitrous oxide accounts for approximately 5% of total emissions, but is approximately 300 times more potent than carbon dioxide (CO2) is as a greenhouse gas.
Nitrous oxide emissions occur from wastewater treatment plants in both the nitrification and denitrification processes, especially high nitrogen strength waste streams, such as recycle streams. Research has shown operational adjustments such as allowing complete nitrification and complete denitrification, or preventing the inhibition of the enzyme nitrous oxide reductase will minimize the formation of nitrous oxide. However, complete removal of nitrous oxide emissions has not been demonstrated.
Given that biological nitrogen removal is becoming more prevalent and has been shown to be the most viable way to reduce nitrogen discharges into water bodies, a method is required to reduce or remove completely the emission of nitrous oxide into the atmosphere.